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When I bought the tank it came with the fish, rocks, plants etc. The fish were 4x neon tetra, 4x harlequins, 4x glow-light tetras and a chinese algae eater.

First off, how did you bring the tank and fish home? I'm assuming the fish were put in a bucket of tank water and the tank emptied for transport? In this case, what did you do with the filter and tank substrate? The filter is what holds your biomedia, whether with ceramic beads, filter sponge ect. The substrate is also very important for this reason. Therefor, if EITHER became dry or dried out, your biomedia could've died. When that happens the tank must be fully re-cycled to restore it to it's previous state as creating live biomedia is the point of cycling.

Originally Posted by jwal90

I wouldn't say there's an issue with the size, I picked out the smallest angel fish I could find and he's only around 3 times bigger than the tetra.

This shows a lack of research to your chosen fish species. Angelfish get quite large, they can get to be anywhere from 6-8" NOT including finnage.

Originally Posted by jwal90

I did changed one of the filter pads but it doesn't seem to have caused any issues. I also added some 'general tonic' which came with the tank when I bought it because it was supposed to help with diseases etc.

Again, please refer to my above mention of the filter holding your biomedia. Ideally you do NOT want to change out your filter pad, instead, during a water change, save a bucket of tank water to rinse out your filter sponge or pad depending on your model. Changing the filter pad out REMOVES the live biomedia and therefore, takes your tank back to step one for cycling as if a fresh tank. It is okay to completely replace charcoal however.

All in all, I believe your tank, though it was previously established, is back to being a new tank that has yet to cycle. DO NOT buy anymore fish and instead IMMEDIATELY get your hands on a water testing kit, liquid drops are more reliable than the strips. Monitor your tank until reach the cycled parameters of ammonia: 0, nitrite: 0, nitrate: 5-15. Those are the basic parameters of a cycled tank.